Co-pending, concurrently filed patent application Ser. No. 819,037, now U.S. Pat. No. 4,752,781 entitled "Side-Looking Airborne Radar (SLAR) Antenna" by the same inventor discloses a radar antenna array which, for mechanical reasons, required coupling between two waveguides separated by a 0.4 inch thick wall and a range of coupling of between -31 dB and -14 dB. But again for mechanical reasons, it was not possible to realize the degree of coupling by the prior art methods of displacing the coupling slot closer to or farther away from the centre line of the wall of the power feeding waveguide. A new composite coupling aperture (conduit actually) was devised to adjust the degree of coupling while accommodating the necessary mechanical constraints.